Currently, there exists a need to reduce the sulfur and aromatics content of motor fuels, in particular diesel, to meet current environmental emission regulations. New “ultra-low-sulfur” diesel specifications are being implemented in the United States Europe and Japan. Under these new regulations, it is proposed that the sulfur level in diesel fuels be reduced to below 0.005 wt. % sulfur, while future regulations may go below this maximum sulfur level. Therefore, many methods have been proposed for producing low sulfur diesel fuels such as, for example, using high pressure reactors, feed undercutting, reducing run lengths, and utilizing high activity hydrodesulfurization catalysts.
However, each of these methods has certain drawbacks. For example, while both the sulfur and aromatics content of diesel boiling range feedstreams from which diesel motor fuels are derived can be reduced to a satisfactory level through the use of catalytic treatments, the catalytic treatments are severely impeded by nitrogen-containing compounds present in the feedstream. Further, conventional hydrodesulfurization catalysts are typically not efficient at removing sulfur from compounds where the sulfur atom is sterically hindered such as those sulfur atoms in multi-ring aromatic sulfur compounds.
U.S. Published Patent Application 2005/0029162 describes methods for desulfurization of naphtha feedstocks having a boiling range from 50° F. (10° C.) to 450° F. (232° C.). The methods include optionally contacting a naphtha feedstock with an acidic material to remove nitrogen compounds, contacting at least a portion of the resulting stream under hydroisomerization conditions with a zeolite having an alpha value between 1 and 100, and then performing a selective desulfurization on at least a portion of the hydroisomerized stream. The methods produce gasoline with improved octane, by a mechanism which appears to at least partially involve converting straight chain olefins into branched olefins. It is believed that this improves octane both because branched olefins are less likely to become saturated, and because any saturation of branched olefins that does occur still results in a branched paraffin, which has a higher octane value than the corresponding straight-chain paraffin.
U.S. Published Patent Application 2005/0023190 also describes methods for desulfurization of naphtha feedstocks having a boiling range from 50° F. (10° C.) to 450° F. (232° C.). The methods include optionally contacting a naphtha feedstock with an acidic material to remove nitrogen compounds, contacting at least a portion of the resulting stream under hydroisomerization conditions with a zeolite, and then performing a selective desulfurization on at least a portion of the hydroisomerized stream. The methods also produce gasoline with improved octane by a mechanism which appears to at least partially involve converting straight chain olefins into branched olefins.
U.S. Published Patent Application 2005/0023191 describes methods for desulfurization of naphtha feedstocks having a boiling range from 50° F. (10° C.) to 450° F. (232° C.). The methods include optionally contacting a naphtha feedstock with an acidic material to remove nitrogen compounds, and then contacting the feedstock with a supported catalyst including at least one medium pore zeolite, at least one Group VI metal, and at least one Group VIII metal. The methods produce gasoline with improved octane, by a mechanism which appears to at least partially involve converting straight chain olefins into branched olefins.
U.S. Pat. No. 6,063,265 provides a process for deep desulfurization of gas oils. The process requires the use of a catalyst that includes a Group VIB metal, a Group VIII metal, and phosphorous.
U.S. Pat. No. 5,897,768 provides a process for improving the desulfurization of petroleum feeds containing hindered dibenzothiophenes. The method includes treating a petroleum feed with a hydrodesulfurization catalyst under hydrodesulfurization conditions and with a solid acid catalyst under isomerization and/or transalkylation conditions.
What is needed is a process that provides further improvement in the speed and efficiency of removal of sulfur from diesel feedstocks.